At the top of a steel tower rising from the floor of the Los Alamos, New Mexico, desert sat an instrument of death the world had never seen. It was just before dawn on July 16, 1945. A test called Trinity, part of a World War II (1939-1945) project to build an atomic bomb, was about to begin.The need for this effort, code-named the Manhattan Project, had arisen 12 years earlier. In January 1933, Adolf Hitler became dictator of Germany. Both his power and his prejudice were enormous. As Germany prepared for war, many brilliant Jewish scientists, fearing persecution, fled the country.By 1939, German troops where overrunning Europe. Afraid that Hitler might be developing an atomic bomb, a group of refugee scientists persuaded Albert Einstein, himself a refugee from Germany, to write to President Franklin D. Roosevelt, explaining the dangers of atomic weapons. After Einstein wrote a second letter in March 1940, small grants of money were made available for scientists to begin atomic experiments in the United States. These tests lead to the first nuclear chain reaction and the conclusion that an atomic bomb was possible, But the question remained: How to build it?

Building a bomb unlike any other was a difficult task. Two very different groups, the U.S. Army Corps of Engineers and scientists, had to work together. The army took care of producing and gathering materials while the scientists experimented with radioactive material.

Unique machinery was designed, plants to produce the plutonium and uranium were constructed, and mini-cities to house the workers were built. At its peak, the Manhattan Project involved more than half a million people.

Work continued throughout 1943 and 1944, and the technical problems were solved one by one. Perhaps the most serious question of all--how large would the explosion be?--remained unanswered until the morning of July 16.

The bomb sitting atop the tower in the desert depended on an inward blast of high explosives to drive two hemispheres of plutonium together, creating an explosion. Soldiers and scientists waited in concrete bunkers as the countdown echoed across the desert.

An intense light, brighter than the sun, turned the predawn darkness into day. A cloud of smoke and fire climbed thousands of feet in seconds, changing color as it rose. Towering miles high in the sky, the cloud looked like a huge, grotesque mushroom. Trinity was a success. It had cost two billion dollars, but the Manhattan Project had reached its goal.

Germany had surrendered in May, but its ally, Japan, continued to fight. Wanting to avoid the loss of any more American lives, President Harry S. Truman decided to use the bomb to convince the Japanese to surrender. On August 6, 1945, a B-29 airplane dropped an atomic bomb on the Japanese city of Hiroshima. Three days later, another bomb destroyed Nagasaki. An estimated 150,000 people were killed, and many more later died from injuries or illnesses related to the bombs' radioactivity. The Japanese surrendered several days later, bringing World War II to a close.​

Upon hearing the news of Hiroshima, Albert Einstein uttered a cry of despair. The Atomic Age had arrived in a way that even the brilliant scientist could not have predicted. How to harness this powerful invention for good has been a question that the world still wrestles with, more than 50 years later.